1. Field of the Invention
The present invention relates to a technique of ensuring a high detection accuracy of both a concentration and a remaining amount of a liquid reducing agent when a moving vehicle mounts thereon a sensor that outputs a signal related to the concentration and the remaining amount of the liquid reducing agent based on a heat transfer characteristics between two points distant apart from each other.
2. Description of the Related Art
As a catalyst purifying system for removing nitrogen oxides (NOx) included in engine exhaust emission, there is proposed an exhaust emission purifying apparatus disclosed in Japanese Unexamined Patent Publication No. 2000-27627 (JP-A-2000-27627). In this exhaust emission purifying apparatus, a reduction catalytic converter is disposed in an exhaust system of an engine and the reducing agent is injection-supplied to an exhaust emission at a position on an upstream side of the reduction catalytic converter thereby catalytically reduction-reacting NOx in the exhaust emission with the reducing agent to purify and turn NOx into harmless components. The reducing agent is stored in a liquid state at room temperature in a storage tank and is injection-supplied from an injection nozzle with a required amount depending on an operating state of the engine. For the reduction reaction, ammonia with satisfactory reactivity with NOx is used. As the reducing agent, a urea aqueous solution, which hydrolyze due to heat of exhaust emission and water vapor to easily generate ammonia, an ammonia aqueous solution, and other reducing agent aqueous solutions (hereinafter, referred to as “liquid reducing agents”) are used.
However, in the above conventional exhaust emission purifying apparatus, when reduction reaction efficiency is changed as the concentration of the liquid reducing agent is varied and if the engine operation is continued without acknowledging this change, NOx reduction efficiency is decreased and the required NOx purifying performance may not be obtained. Especially, NOx may be emitted in a large amount if the engine operation is continued while a mixture ratio between the reducing agent and water both constituting the liquid reducing agent is inappropriate, a different kind of aqueous solution or water is mixed, or a remaining amount of the liquid reducing agent becomes insufficient.
Therefore, provision of a sensor for detecting a concentration and a remaining amount of the liquid reducing agent based on a heat transfer characteristic between two points distant apart from each other is conceivable. However, when this sensor is mounted on a moving vehicle such as an automobile, the following problem might occur. Namely, because a vehicle body constantly vibrates due to undulations of a road surface during driving of the mobile vehicle, the liquid reducing agent can be moved to cause a phenomenon of convection in the storage tank. When the convection of the liquid reducing agent occurs, the heat transfer characteristics through the agent which behaves as a heat transfer medium changes, and thus, the accuracy in detection of the concentration which is more sensitive to the change in the heat transfer characteristics than the remaining amount of the liquid reducing agent must decrease significantly.